Brushless axial flux PM electrical machines (motors and alternators) have been known for many years and embodied in many practical machines. The most common types are an axial version of a salient pole brushless permanent magnet machine. Such rotors feature magnets bonded onto (surface PM type) or embedded in (embedded PM type) an iron or electrical steel rotor structure. The stator windings are wound around steel poles or teeth comprised of stacked electrical steel laminations. A variant on this design would be the “slotless” design that retains an electrical steel “yoke” behind the windings to help complete the rotor excitation magnetic circuit, but does not have steel poles, teeth, or slots that the windings fit into. Somewhat less common are axial flux machines that feature no steel laminations. The windings may also be formed as a printed circuit board or punched from copper sheet. The variety of winding geometries interact with an annular disk of magnets that cause a changing flux through the coils as the annular disk of magnets rotate relative to the windings or vice versa. Even less common are “ironless” designs that feature a non-magnetic rotor structure and/or stator structure.
It is well known that in high speed permanent magnet machines special care needs to be taken to ensure that the magnets stay bonded onto the rotor in the case of a surface PM machine. It is also known that centrifugal stresses in the rotor may cause the magnets to crack because the magnet material is brittle and may not tolerate much tensile stress. Both surface and embedded PM machines must deal with centrifugal loading stresses.
In many high speed radial flux machines such as flywheel alternators, a steel band is applied outside the magnets using an interference fit. This applies static compressive stress into the magnets and the rest of the rotor material so that even under the centrifugal loading the magnets never go into tension. The interference fit usually requires very high tolerances on the two parts to ensure that sufficient compressive preload is generated without over-stressing any components. U.S. Pat. No. 4,638,200 issued to Le Corre et al. on 20 Jan. 1987 provides an example of this technique. This same technique has been applied to axial flux machines.
Other high speed radial flux machines utilize a carbon fiber wrap over the magnets on the outside diameter of the rotor. When the rotor spins quickly the high modulus fiber may act as an outer containment that prevents the magnets from coming off of the steel rotor. This technique is difficult to implement in practice as the carbon fiber wraps needs a significant preload/pre-stress tension to ensure that it carries the tensile load rather than the bonds of the magnets to the steel rotor. U.S. Pat. No. 6,047,461 issued to Miura et al. on 11 Apr. 2000 provides an example of this technique.
Another approach is shown in prior art FIG. 1a and related FIG. 1b which is a cross section taken along line 1b-1b in FIG. 1a. Here, the approach is that of an alternating pole machine using magnets 40, 42 embedded into the material of the rotor plates 30, 32 on a shaft 16 with gaps between the magnets. This is a well understood structural design as the optimal arrangement of magnets for an alternating magnet rotor has gaps between the magnets. Putting structural material in these gaps is a well understood way to strengthen the rotor beyond the strength of the backing plate 34 behind the magnets. A rim 36 and restraining band 38 may, as before, be provided. Typically, this approach uses round magnets that have large gaps between them, although some examples using trapezoidal alternating pole magnets are known as well. U.S. Pat. No. 4,996,457 issued to Hawsey et al. on 26 Feb. 1991 provides an example of this technique with round magnets with alternating poles. The holes that the magnets are placed into do not have top or bottom surfaces on them.
Another approach is shown in prior art FIGS. 2a and 2b which provides an example of a known axial flux machine with alternating pole trapezoidal magnets 23 where a magnet locating and retaining device 1 is added to the magnetic rotor structure. This design may be understood as a hybrid of the surface mount PM and interior PM designs. By way of pins 37, apertures 39, and connecting clips 35, the retaining device assists in keeping the magnets attached to the rotor in the event of an adhesive failure, but the retaining device does not provide the primary structural restraint to carry the centrifugal loads from the magnets to the rotor hub. U.S. Pat. No. 8,598,761 issued to Langford et al. on 3 Dec. 2013 provides an example of this technique.
A notable concept in the advancement of motor design is the notion of the Halbach array. This is an array of magnets which orientations are chosen to focus magnetic field on one surface (called the strong or active surface) with the field on the other surface being much reduced (called the weak or inactive surface). As shown by way of FIG. 3, linear Halbach arrays are known and which focus fields on one side of a surface. U.S. Pat. No. 5,705,902, herein incorporated by reference, issued to Merritt et al. on 6 Jan. 1998 provides an example of this technique. A Halbach array may also be arranged in an annular ring such that the active surface faces along the axis of the annulus.
Halbach arrays of magnets are usually designed without gaps between the magnets to ensure that the magnetic field generated by the array has the highest magnitude possible; and also to minimize the harmonic content of the sinusoidal Halbach field. Sometimes, curved Halbach arrays are assembled out of rectangular parallelepiped for cost-saving purposes in which case there are “wedge shaped” gaps between the magnets that are sometimes filled with structural material if the array is built in a “pocketed structure.” This is sometimes done with a Halbach array forming a cylinder with a radial field. Wedge shaped structures have stress concentrations and are suboptimal from a structural standpoint.
In axial flux Halbach machines, the magnets for the array are trapezoidal shape and are often bonded onto a rotor plate structure that is “behind” the array on the inactive surface—that is on the side of the array axially displaced opposite the machine active air gap. The trapezoidal shape of the magnets minimizes the gaps between magnets and maximizes the magnetic field of the array. Less expensive rectangular magnets may be used, but this results in a lower magnetic field and large wedge shaped gaps between the magnets at the periphery of the rotor. The adhesive bond in such a design is subject to failure at high rotor speeds. Prior art FIG. 4 shows one example of a Halbach array rotor structure 10 for an axial flux machine where the trapezoidal magnets (12A-1, 12B-1, 12A-2, 12B-2 and so on) are bonded to one another to form an annular ring 12.
A dual Halbach design is one where the active surfaces of two Halbach arrays are directed at a single winding. The rotor plates in a dual Halbach array axial flux motor must be designed to withstand the attractive axial forces from the magnet arrays and also centrifugal forces caused by the spinning of the rotor. Also the attractive forces deform the rotors and alter the magnetic gap. Depending on the detailed shape of the rotor, the centrifugal force may cause expansion, convex cupping, concave cupping and other distortion of the annular rotor. Prior art FIG. 5 shows such misalignment of the centrifugal forces 501 on the magnets 510 and the opposite restraining forces from the rotor structure 502. This misalignment of forces creates a bending moment within the rotor structure that causes the rotor to distort to a cup shape at high speeds as shown in the dashed line areas 510a. These distortions must be minimized to maintain the gap in the motor. Further, any deformation of the rotor may apply stresses to the magnets which generally have modest strength and are brittle.
Halbach arrays are used in electrical machines that are lightweight. To this end, the magnets are a significant portion of the mass of the machine rotor. When the Halbach array of magnets is bonded together into an annular ring, the solid ring becomes very stiff relative to the lightweight materials used in the rotor structures such as titanium or aluminum. When the machine rotor is spun to high speeds, a hoop stress is experienced by the rotor materials. Since the relatively thick bonded magnet assembly is significantly stiffer than the thin, lightweight rotor plate all of the hoop stress will be carried by the magnet assembly.
Hoop stress due to centrifugal acceleration in a thin rim of material is known to be computed as σt=ρ×velocity2 where σt is the tensile stress in the hoop, ρ is the density of the hoop material, and velocity is the surface speed of the cylinder. Based on the density of neodymium iron boron magnets of 7.5 gm/cm3, it may be computed that at a surface speed of 100 m/s the hoop tension stress in the annular magnet ring assembly will be approximately 7.5*107 Pa (10,880 psi) which far exceeds the tensile bond strength of most epoxies and is approximately equal to the tensile strength of the sintered neodymium iron boron magnet material. The above calculation is greatly simplified, but when the more complicated thick wall calculations are done the stresses are found to be even higher. High speed electrical machines typically have surface speeds even greater than 100 m/s. Thus, at any reasonable speed for a machine categorized as “high speed” with magnets in a solid annular ring and a lightweight rotor, any bonds between the magnets will break and even the magnets themselves are subject to breaking.
Because the magnets may not withstand the centrifugal tensile forces from high speed operation, those forces must instead be taken by the rotor structure such as the backing plate that the magnets are bonded to. If the rotor structure is very thick and heavy relative to the magnets, there will only be a small strain in the rotor material. However, if the rotor structure is thin and lightweight, then the rotor materials must undergo a significant strain as they will be under significant centripetal tensile stress.
The magnets, however, are brittle and may not undergo significant strain without cracking. These magnets are bonded to the rotor structure, which is highly stressed and strained in a lightweight machine. Thus, the bond layer between the magnets and the rotor structure on the inactive surface of the Halbach array will likely fail or the magnets will end up being cracked as the rotor structure stretches and experiences strain due to the centrifugal loading.
These computations show that it is difficult to make an axial flux Halbach array rotor structure that is lightweight and which structurally withstands operation at surface speeds greater than 75 m/s.
Additional structural problems arise once the magnet structure is no longer a solid ring but is a collection of separated masses attached to the rotor structure. In a surface PM machine, the mass of the magnets is not located along the centroid of the rotor structure with regards to the centrifugal forces on the magnets as the magnets are mounted on the surface of the rotor structural member.
It is therefore desirable to overcome the aforementioned problems associated with the prior art.